£45kpa for one person is less net than £45k pa joint as you only get one lot of (£12k from April) tax free allowance. If two people earn £23k each then they each have £12k tax free and only pay tax on £11k each, so £22k total.
The single £45k earner pays tax on £33k.
The take home is what matters.
The OP has apparently had £1,400 to live on after mortgage - so presumably has some savings? If they have been unable to save at all on that then no, they will not be able to live on it after £1k pm is allocated to child care.
The usual options:
reduce outgoings (change mortgage is the obvious one, has to be done while still working, cheaper child care etc)
increase incomings (ask for pay rise? change job? take a lodger? double check how tax credits work. add in the child allowance. sell some stuff now to get a buffer pot?)
Also as a PP said, live on that £400 now, save the £1k and see how it feels. Do that for 6m then try for the pregnancy if it has worked OK. By the time the baby comes you'll have £15k towards the things you need and topping up mat pay.
Surely one does not wake up one morning thinking "I need to have a baby, today, can I afford it, oh no, it's so unfair" - surely one knows they might want to have a baby one day and plan and save for that? Or save for any other reason?
It does actually sound doable to me, if it is wanted desperately enough.